gh-152056: Compile single-category character sets to a bare CATEGORY opcode (GH-152057)

gh-152056: Compile single-category character sets to a bare CATEGORY opcode (GH-152057)

A character set containing exactly one category, e.g. [\d] or [^\s], now
compiles to a single CATEGORY opcode (like \d or \S) instead of an IN
block.  The negated form maps to the complementary category.  This speeds
up matching and reduces the size of the compiled byte code.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>